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bug#50110: [External] : Re: bug#50110: 26.3; Doc of `assoc': TESTFN opti
From: |
Drew Adams |
Subject: |
bug#50110: [External] : Re: bug#50110: 26.3; Doc of `assoc': TESTFN optional arg |
Date: |
Wed, 18 Aug 2021 18:41:00 +0000 |
> > Neither the `assoc' doc string nor the Elisp manual, node `Association
> > Lists' explains the order of the args that get passed to TESTFN.
>
> It's a function that tests 2 objects for equality, so why does the
> order matter?
>
> > It's just as reasonable to expect that the pattern is the first arg to
> > TESTFN, instead of, as is actually the case, the second arg.
>
> There's no "pattern" in the doc string, so I don't think I understand
> what you are describing here.
I gave examples. What part is unclear? To know how to use
`string-match-p' for testing you need to know the arg order
for TESTFN. The arg order matters for `string-match-p'.
IMO, it's an arbitrary predicate that's applied to 2 args,
one of which is the car of an alist element. It's not
necessarily an _equality_ predicate. You're may disagree,
but I think it would be more helpful to document which
TESTFN arg corresponds to the alist element cars.
FWIW, here's a user who ran directly into this doc problem,
which motivated this doc bug report:
https://emacs.stackexchange.com/q/68172/105
FWIW2, Common Lisp specifies the order, and in fact it is
the reverse of the order used by TESTFN in Elisp. See here:
https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node141.html#SECTION001800000000000000000
This text:
In the following function descriptions, an element x of a
sequence "satisfies the test" if any of the following holds:
* A basic function was called, TESTFN was specified by the
keyword :test, and (funcall TESTFN item (KEYFN x)) is true.
This would mean you would pass just predicate `string-match-p',
not (lambda (x y) (string-match-p y x)), as TESTFN for the
example. (Here, KEYFN would be the default, `car'.)